According to the Technical Requirements: The "_color" texture can be 24 or 32 bit. Use 32 if the alpha is needed for opacity So simply apply a 50% grey alpha (I'd go a bit lighter, 50% may still be too little) where you want it to be semi-transparent and save the diffuse color as a 32 bit.
PoPcorn - Bro, i got that ERROR yesterday. Its because one your masks have 24 bit format. double check your maps diffuse = 24 bit (32 bit if you are using alpha or transparency) Normal = 24 bit mask 1 = 32 bit mask 2 = 32 bit. Please check this and import. Hopefully it should work out.